This invention relates to apparatus for cutting receptacles from sheets of heat weldable material. More particularly, this invention concerns the unstripped cutting and hermetic sealing of irregularly shaped receptacles from sheets of heat weldable material.
In the past, receptacles fabricated from heat sealable synthetic resinous materials, such as polyethylene, have been widely used to package and store various objects and commodities. Some receptacles are generally rectangular and are fabricated from a tubular piece of heat sealable material. Other generally rectangular receptacles are fashioned from overlying sheets of heat sealable material.
To fabricate the generally rectangular receptacles, an essentially straight heated blade is often used. The heated blade first seals a straight edge of the receptacle and, when all the necessary edges are formed, a separate guilotine apparatus severs each receptacle from the substantially continuous sheets of material.
Typically, the heated blade has an internal heat source that is not easily shaped in complex contours. Accordingly, the heated blades are ordinarily linear. Moreover, each heated blade is usually provided with a separate heat source to provide control of its temperature.
Irregularly configured receptacles, as opposed to circular or rectangular receptacles, are used, for example, as novelty items and as liners in irregularly contoured vessels. Because of the irregular shape, known heated blades are not truly suitable for fabrication of these complex shapes.
One approach used in the cutting of irregularly shaped receptacles from sheet material is to mount a resistance wire on a plywood support. The supported wire is placed in contact with the sheets of heat weldable material. Then an electrical pulse is passed through the wire thereby heating it and allowing the heated wire to cut through the sheets of material.
With the hot wire apparatus, precise dimensional tolerances are difficult to obtain since the hot wire undergoes thermally induced linear expansion when the wire is pulsed. Moreover, the wire wears out quickly. A particularly disadvantageous feature of the hot wire apparatus concerns the fact that it actually strips or cuts out an irregular shape from the sheet material. In this fashion, each of the irregular shapes, as well as the parent sheet material, must be handled during fabrication and a concurrent expense is experienced. Additionally, the hot wire gives a receptacle an erractic edge configuration which typically has imperfections such as pin holes through which fluids can escape from the receptacle.
In the tire packaging art, concentric annular members have been used to separately cut and seal plastic packaging material around annular object prior to heat shrinking of the material to the object. An example is illustrated by U.S. Pat. No. 3,868,291 issued to Benz et al on Feb. 25, 1975. Apparatus for packaging solid objects is not concerned with providing an hermetic seal. Moreover, annular heating elements are not readily deformed to complex irregular shapes.
Where separate sealing and cutting operations are provided, they are ordinarily independent, rather than interdependent. Moreover, the sealing operation is not guided by the cutting operation to allow precise control of the spacing between the cut and seal lines. Accordingly, indexing of the separate cutting and sealing operations is required.
With known apparatus for simultaneously cutting and sealing, such as hot wire and heated blades, pin holes and similar imperfections occur along the cut line. Thus, an hermetic seal is not present. These pin holes are particularly objectionable as fluids may leak from the receptacle thereby rendering the receptacles unsuitable for use with fluids or, for that matter, sterile objects.
Thus, it will be seen that the need continues to exist for a truly effective apparatus capable of fabricating unstripped hermetically sealed irregular objects from heat sealable plastic materials.